He said it appeared the aircraft had "suddenly dropped into the sea without any opportunity to make a controlled landing".

Amanda Smith, whose son Sam was on the helicopter, told Sky News it suddenly lost power and those on board had "no time to brace".

"He was by the window so he was able to escape that way as it rolled over," she said.

"He said he had come off better than a lot of people, [those] were his words."

PREVIOUS NORTH SEA INCIDENTS

October 2012 - All 19 people on board a Super Puma EC 225 were rescued safely after it put down in the sea off Shetland. The incident was caused by a cracked shaft in the main gearbox.

May 2012 - All 14 people on board a Super Puma EC 225 were rescued when it came down about 30 miles off the coast of Aberdeen during a flight to an oil rig.

April 2009 - All 14 passengers and two crew on board a Super Puma AS332L2 lost their lives after it came down in the North Sea. Eight of the victims came from the north east of Scotland, seven from the rest of the UK, and one from Latvia. A fatal accident inquiry is planned for October.

February 2009 - A Super Puma EC225 ditched in fog a short distance from a BP oil platform in the ETAP field, 125 miles east of Aberdeen. All 18 people on board survived. Crew error and a faulty alert system were blamed.

Michael Bull, whose son Samuel was rescued, said: "We understand he was on his way back from a rig and the helicopter lost power suddenly and immediately ditched into the water.

"He managed to escape straight away because he was right by an exit and I understand soon afterwards that the helicopter turned over."

A total of 18 people were on board the helicopter.

The 14 survivors, including the two crew members, were taken to Gilbert Bain Hospital in Lerwick for treatment.

Police Scotland said five were discharged a short time later and nine were detained overnight either for observation or suffering from exposure.

The ditched helicopter was found broken into several pieces up against rocks.

Boats, including a ferry and a cargo ship, joined lifeboat crews from Lerwick and Aith and helicopters from the coastguard, RAF Lossiemouth and two Bond rescue helicopters to search for survivors.

The AS332 L2 helicopter, carrying passengers and crew from the Borgsten Dolphin oil rig in the North Sea, was operated by CHC for Total, taking people to and from oil and gas installations.

A CHC spokesman said: "The aircraft was on approach to Sumburgh Airport at approximately 6.20pm when contact was lost with air traffic control."

In a later statement, the company said the cause of the incident was unknown but Super Puma L2 flights would be suspended worldwide.

"Also, in deference to the incident and the investigation, we are suspending all flights [on] Saturday by our UK operations," the company added.

Bond Offshore Helicopters also said it would not be operating any of its Super Puma aircraft fleet, with the exception of its Jigsaw rescue aircraft which would be available for life at risk missions.

Oil firm Total confirmed that the three men and one woman who died all worked for contract organisations.

Stork Technical Services confirmed one its employees, Gary McCrossan, was one of those who had died.

Mike Mann, a senior vice president at the firm, said: "Our heartfelt condolences go out to Gary's family and to all of those affected by this tragedy. We are doing all we can to assist the family at this difficult time."

Safety training

Scotland's First Minister Alex Salmond paid tribute to the "brave and hard-working" people involved in the rescue effort.

• NORTH SEA SURVIVAL TRAINING

All offshore workers are required to undertake offshore safety training, such as emergency first aid, sea survival and helicopter underwater escape training.

Core courses include Minimum Industry Safety Training (Mist) and Basic Offshore Safety Induction and Emergency Training (Bosiet). Bosiet covers emergency responses on an offshore installation as well as during helicopter transit. The course includes time in a helicopter simulator in a swimming pool.

Offshore personnel can also undertake Further Offshore Emergency Training (Foet).

A valid offshore medical certificate of fitness is required for those working in the North Sea.

He added: "Our thoughts at this difficult time are with the families, friends and colleagues of those who lost their lives in this tragic incident."

Oil & Gas UK's chief executive, Malcolm Webb, also offered his condolences, and said the incident had emphasised the importance of safety training.

He added that helicopter safety remained a focus for the industry, and any lessons that could be learned would be shared across the industry.

Last year, Super Puma helicopters crashed in two incidents, one off Aberdeen and another off Shetland, but these involved the EC 225 variety of the aircraft.

All passengers and crew were rescued in both incidents which were found to have been caused by gearbox problems.

Super Puma EC 225s were grounded following the crashes but were given the go-ahead to resume flying again earlier this month.

Bob Crow, general secretary of offshore union RMT, said there was a lack of workforce confidence in the Super Puma type aircraft, and unions had been working with the industry to address their members' concerns.